Processes that contact reactants with circulating catalyst particles in a reaction zone are well known. One of the most well known processes that contacts reactants and regenerants with circulating catalyst particles is the fluidized catalytic cracking (FCC) process for the conversion of heavy hydrocarbons. U.S. Pat. No. 3,844,973 shows a regenerator arrangement used to regenerate catalyst in an FCC process that has a first dense bed that supplies catalyst to a relatively dilute phase catalyst mixture in a superadjacent transport riser. U.S. Pat. No. 3,919,115, U.S. Pat. No. 3,953,175, and U.S. Pat. No. 4,340,566 shows a variety of additional FCC regenerator arrangements that operate with a relatively dense phase bed and that supply catalyst particles to a relatively dilute phase transport riser.
Processes for the ammoxidation of propylene to produce acrylonitrile are generally well known. U.S. Pat. No. 4,246,191 provides an extensive list of references and specific descriptions of various patents that describe different methods of contacting reactants for the production of acrylonitrile with particulate catalyst for carrying out the ammoxidation reaction. U.S. Pat. No. 4,246,191 is particularly directed to temperature control of particulate catalyst in a fixed bed reaction zone having a top bed surface that extends close to the inlets of devices for the separation of catalyst from the acrylonitrile products. Control of the temperature in the particle bed minimizes the temperature deviation along the reactor profile.
The ammoxidation of propene to produce acrylonitrile is generally believed to be a redox process and that lattice oxygen from circulating solids can supply oxygen to the reaction. It is also known that lattice oxygen can be regenerated by air at certain temperature ranges. U.S. Pat. No. 4,152,393 discloses a reactor design for circulating catalyst from a first fluidized bed for the ammoxidation of propylene to produce acrylonitrile and a second bed for the regeneration of the catalyst. The arrangement continuously circulates catalyst from one bed to another and isolates the fluids from each individual bed to prevent intermixing. U.S. Pat. No. 4,246,192 teaches the oxidative regeneration of catalyst for the ammoxidation of olefins by the taking of a small stream of catalyst from a reaction zone for the ammoxidation of olefins. In this arrangement catalyst transfer lines connect the regeneration zone and reaction zones in this arrangement.
The ammoxidation reaction may also be carried out without a separate regeneration zone in which case oxidation of the catalyst material occurs within the reaction zone. Typically, a stoichiometeric excess of oxygen in relation to the feed gas will be sent to the ammoxidation reaction zone.
Single bed reactors for the ammoxidation of propylene to produce acrylonitrile have been generally preferred. In particular the fixed bubbling bed type have been preferred to reduce the volume of catalyst circulation required in conventional transport-type arrangements for ammoxidation of propylene. (See page 14 of "New Developments in Selective Oxidation II". Proceedings of the second World Congress and Fourth European Workshop Meeting, Benalmadena, Spain, Sep. 20-24, 1993). The use of multiple beds of the same type, again the bubbling bed variety, is disclosed in a paper by the CHEMISCHE TECHNIK 48 (1996) titled "Effect of Hydrodynamic Conditions in an Industrial Scale Fluidized Bed Reactor on Selectivity and Yield in Acrylonitrile Preparation by Ammoxidation of Propene.
The ammoxidation of propene typifies heterogenuous catalytic reactions that evolve large amounts of heat, but require a relatively narrow range of temperature to be maintained as the reactants contact the particulate catalyst in order to inhibit secondary reactions that produce unwanted by-products. In the case of ammoxidation the reaction is very exothermic and continued reaction can further oxidize the product into nitrogen and carbon oxides when in the presence of oxygen. As demonstrated by the prior art, reactors for ammoxidation of propylene to produce acrylonitrile are well known. The current reactor configuration favored for most commercial arrangements is a bubble fluidized-bed reactor. The superior heat transfer characteristics of the bubble fluidized-bed reactors have led to their wide spread acceptance for ammoxidation. The reactor removes the reaction heat from the ammoxidation reaction efficiently by installing a heat transfer device inside the reactor.
A long standing problem with the use of a bubble fluidized-bed reactor is the relatively low superficial gas velocity that can be maintained through the reactor while still operating in a dense phase condition. Superficial gas velocity through the bed must be restricted to around 0.5 m/s. These low velocities present serious back-mixing problems within the reactor that randomly changes residence time and overall lowers the selectivity of the process to the desired product.
One solution for reducing the degree of backmixing has been a circulating fluidized/bed (CFB) reactor. This reactor reduces the degree of gas mixing by passing reactants and catalysts to a transport conduit such as a riser-type reactor. Such reaction arrangements promote careful control of reaction times and generally approach a plug-flow contacting of reactants and catalyst particles. A draw back to the CFB-type reaction arrangement is the removal of heat. Heat transfer devices are typically not suitable for use within the circulating particulate catalyst environment. The provision of a heat exchange surface in a CFB-type reaction zone also leads to catalyst attrition as well as presenting poor heat transfer conditions due to the relatively low density of the catalyst contained therein. Therefore, reaction heat has to be removed from the reactor by circulating catalyst to avoid any large temperature increase which, again, will result in a higher yield of undesired by-products such as carbon dioxide. Typically, such reactors require a high catalyst circulation rate to maintain low temperature increases in the reactor. For example, a typical CFB-type reactor must circulate about 800 kg of solid catalyst for each kg of acrylonitrile that is produced to keep the temperature increase below about 10.degree. C. as the catalyst and reactant mixture passes from the inlet to the outlet.
It is known from EP-A-0189261 to use the combination of a bed-type reaction zone below a riser-type reaction zone for the production of maleic anhydride. EP-A-0189261 also discloses that a metal oxide may serve as a carrier for a significant portion of the stoichiometrically required oxygen through rapid and reversible oxidation-reduction. However, EP-A-0189261 teaches, in conjunction with its examples for maleic anhydride production, that riser reaction zones, fluidized bed reaction zones and combinations thereof have no particular advantage over each other.
Therefore, it is an object of this invention to increase selectivity for acrylonitrile by reducing the backmixing inside a particle contact-type reactor.
It is another object of this invention to lower catalyst circulation in a CFB-type reactor arrangement for the production of acrylonitrile.
It is a further object of this invention to control oxidation reactions that take place within a typical ammoxidation reaction.
It is a yet further object of this invention to improve the selectivity of acrylonitrile generated from an ammoxidation reaction.
It is another object of this invention to improve the mixing of oxygen that enters the ammoxidation reaction zone.